This invention relates generally to flue gas conditioning systems and more particularly to an improved method and apparatus for introducing an acid conditioning agent into a flue gas stream for enhancing the removal of finely-divided fly ash particles by electrostatic precipitation.
Many industrial and utility companies employ coal fired boilers in their power plants. In an effort to comply with todays strict emissions standards, a number of these companies have switched to the use of low sulfur coal to reduce the amount of sulfur dioxide present in the flue gases. Unfortunately, the use of low sulfur coal in these boiler plants lowers the amount of sulfur trioxide which naturally occurs in the flue gas stream. The presence of sulfur trioxide is known to develop a sufficiently low resistivity in the fine fly ash particles to promote their efficient removal from the flue gas by electrostatic precipitation.
In an effort to restore the level of sulfur trioxide to that developed in the flue gas when using high sulfur coal, many industrial and utility companies employing coal fired boilers are now resorting to the use of flue gas conditioning systems. These systems are designed to bring the exhaust fly ash resulting from the combustion of low sulfur coal into a range of resistivity which is more desirable for removal by electrostatic precipitation. Gas conditioning is far more attractive from an economic standpoint when compared to the cost of installing new and larger precipitators that would otherwise be required to handle the flue gas resulting from the burning of low sulfur coal.
Various flue gas conditioning systems and methods have heretofore been proposed. For example, L. C. Hardison, et al U.S. Pat. No. 3,704,569, discloses a system which uses vaporized sulfuric acid as a conditioning agent. In this system, large volumes of dry air are heated to a temperature of approximately 260 degrees C to vaporize the sulfuric acid, which is mixed with the air in a glass lined chamber filled with dispersion packing. Since the glass and gasket material that is employed in this system limit the temperature to not more than about 260 degrees C, the acid is evaporated, rather than boiled, in the air stream. The hot vaporized acid is uniformly dispersed in the flue gas stream by means of injection lances, the vaporized acid being conveyed to the lances using glass lined manifold pipes. Although this system provides effective conditioning of the flue gas, it is nevertheless expensive, primarily because the acid must be transported over long distances in a hot vaporized state. The vaporized acid is, of course, extremely corrosive and requires the use of expensive, corrosion-resistant materials. Moreover, the system consumes large amounts of electrical energy in order to heat the large volumes of dry air that are required to evaporate the acid into the air stream.
Gas conditioning can also be carried out by introducing hot, vaporized sulfur trioxide directly into the flue gas stream. However, the sulfur trioxide is extremely difficult to handle since it must be heated to remain liquid and can solidify in piping systems if the heating should fail. When reheated, the solidified material becomes an extremely corrosive gas under high pressure which can rupture the piping. Moreover, the resulting gas forms fuming sulfuric acid with atmospheric moisture. For these reasons, the direct addition of sulfur trioxide would not be a commercially viable process.
Another proposal for gas conditioning is that of burning liquid sulfur. The sulfur dioxide that is generated by burning sulfur is passed through a catalyst which converts the sulfur dioxide to sulfur trioxide. The latter is introduced into the flue gas stream where it combines with the moisture in the flue gas to form sulfuric acid. The sulfuric acid conditions the fly ash as indirect sulfuric acid injection. Another variation is to heat liquid sulfur dioxide to a vapor, pass it through a catalyst that converts it to sulfur trioxide, and disperse the sulfur trioxide in the flue gas as in the aforementioned method.
W. I. Olson, et al U.S. Pat. No. 4,070,424, discloses a flue gas conditioning system utilizing high energy compressed air acoustic atomizing nozzles to produce an extremely fine mist of liquid sulfuric acid which is then vaporized in the surrounding hot flue gas or air. A number of the nozzles are positioned in the inlet duct to the precipitator, which sprays sulfuric acid directly into the flue gas stream. However, the problem with this system is that under normal gas flow conditions, the newly atomized plume of sulfuric acid can collapse and cause reaglomeration of the acid mist into larger droplets which fail to vaporize, wetting the internal duct structure and causing undesirable ash build-up and corrosion.
W. A. Quigley, et al U.S. Pat. No. 4,208,192, discloses a similar system which utilizes high energy compressed air acoustic nozzles to inject a fine mist of acid into a slip stream of hot air where the acid is vaporized in a large cyclonic flow chamber. The resulting vaporized acid/hot air mix is then injected into the inlet duct upstream of the precipitator. The problem with this system is that it requires apparatus which is very bulky and less efficient in the use of vaporizing energy and compressed plant air.
It is therefore an important object of the invention to provide an improved flue gas conditioning system which is more efficient and less costly than the known systems of the prior art.
Another more specific object of the invention is to provide an improved method and apparatus for introducing an acid conditioning agent into a flue gas stream in vaporized form in order to develop a more desirable resistivity in the finely-divided fly ash particles to promote their removal by electrostatic precipitation.